Inorg. Chem. 2001, 40, 4721-4725
4721
Synthesis and Structure of [K+-(2,2)diaza-[18]-crown-6][K3Ge9]‚2Ethylenediamine: Stabilization of the Two-Dimensional Layer 2∞[K3Ge91-] Craig Downie, Jiang-Gao Mao, and Arnold M. Guloy* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 ReceiVed October 4, 2000 Large bright-red, transparent crystalline plates of [K-(2,2)diaza-[18]-crown-6]K3Ge9‚2en are obtained, in highyield, from a reaction of (2,2)diaza-[18]-crown-6 in toluene with a solution of K4Ge9/potassium metal (K) in ethylenediamine (en). The compound crystallizes in the monoclinic space group P21/m (a ) 10.740(1) Å, b ) 15.812(1) Å, c ) 12.326(1) Å, β ) 114.74°; Z ) 2). The crystal structure of [K-(2,2)diaza-[18]-crown-6]K3Ge9‚2en features two-dimensional [K3Ge9] layers formed by uncomplexed K+ cations and Ge94- anions. The “not-so-bare” cluster compound features a unique Ge94- cluster that exhibits a slightly distorted C2V geometry that is closer to D3h than the expected C4V. Use of noncryptand sequestering agents in the isolation of Ge cluster anions from en solutions opens new avenues in understanding important cation-anion interactions in the stability and reactivity of Zintl ions, as well as a viable route to isolating Zintl anions with higher charges per atom.
Introduction The question of how properties of semiconductors evolve as dimensions approach interatomic distances is currently a topic of great scientific interest and technological implications.1-5 Zintl anions of group 14 elements (Ge, Sn, Pb), in particular the germanium nine-atom clusters, offer a unique class of “naked” elemental cluster ions that represent a bridge between small clusters and their corresponding nanocrystalline bulk semiconductors.6 The relevance of Ge9n- anions was recently emphasized by the results of an unbiased global search for the ground-state structures of Si and Ge clusters.2,7 Using molecular geometry optimization calculations, it was shown that the structures of medium-sized Si and Ge clusters are based on tricapped trigonal prism Si9 and Ge9 units.2,5,7 Recently we have reported the synthesis of a novel polymeric [Ge9]2- chain composed of nido-Ge9 clusters linked through their vertexes.8 Moreover, the preparation of similar Ge186- dimer anions lends support to the oxidative coupling of nido-Ge9 clusters. The existence of the polymeric [Ge9]2- cluster chain and the dimeric (1) (a) Brus, L. E.; Szajowski, P. F.; Wilson, W. L.; Harris, T. D.; Schuppler, S.; Citrin, P. H. J. Am. Chem. Soc. 1995, 117, 2915. (b) Heath, J. R. Science 1992, 258, 1131. (c) Jarrold, M. F.; Honea, E. C. J. Phys. Chem. 1991, 95, 9181. (d) Bley, R. A.; Kauzlarich, S. M. J. Am. Chem. Soc. 1996, 118, 12461. (2) Ho, K. M.; Shvartsbyrg, A. A.; Pan, B.; Lu, Z.-Y.; Wang, C.-Z.; Wacker, J. G.; Fye, J. L.; Jarrold, M. F. Nature 1998, 392, 582. (3) (a) Vasiliev, I.; O ¨ gut, S.; Chelikowsky, J. R. Phys. ReV. Lett. 1997, 78, 4805. (b) O ¨ gu¨t, S.; Chelikowsky, J. R. Phys. ReV. 1997, B55, R4914. (4) Hunter, J. M.; Fye, J. L.; Jarrold, M. F. Phys. ReV. Lett. 1994, 73, 2063. (5) Lu, Z.-Y.; Wang, C.-Z.; Ho, K.-M. Phys. ReV. 2000, B61, 2329. (6) (a) Corbett, J. D. Angew. Chem., Int. Ed. 2000, 39, 671. (b) Corbett, J. D. Structure Bonding 1997, 87, 157. (c) von Schnering, H.-G. Angew. Chem., Int.Ed. Engl. 1981, 20, 33. (7) (a) Shvartsburg, A. A.; Jarrold, M. F.; Liu, B.; Lu, Z.-Y.; Wang, C.Z.; Ho, K.-M. Phys. ReV. Lett. 1998, 81, 4616. (b) Shvartsburg, A. A.; Liu, B.; Lu, Z.-Y.; Wang, C.-Z.; Jarrold, M. F.; Ho, K.-M. Phys. ReV. Lett. 1999, 83, 2167. (c) Liu, B.; Lu, Z.-Y.; Wang, C.-Z.; Ho, K.-M.; Shvartsburg, A. A.; Jarrold, M. F. J. Chem. Phys. 1998, 109, 9401. (8) Downie, C. D.; Tang, Z.; Guloy, A. M. Angew. Chem., Int. Ed. 2000, 39, 338.
[Ge18]6- anions provides support to the theoretical cluster models associated with larger Ge clusters based on tricapped trigonal prism units.8,9 Thus, oxidative coupling of Ge cluster anions represents a viable chemical route to the controlled growth of larger Ge clusters and nanocrystals. Likewise, this has significant implications to the cluster chemistry of silicon.7 A number of homopolyatomic germanium Zintl cluster anions have been previously isolated from basic solution.8-14 The general route to their isolation from ethylenediamine solution has been the use of strong sequestering agents for alkali metal ions, as in [2.2.2]cryptand. Recent investigations on the nature of Ge9n- clusters in ethylenediamine (en) have addressed the validity of charge allocation assignments among Ge9n- (n ) 2-4).10-12,15 These investigations posed questions on the existence of closo-Ge92- and the geometry of nido-Ge94-. Thus, it became important to unambiguously isolate the closo-Ge92and nido-Ge94- anions from appropriate alloy-en solutions. Previous investigations show that en solutions of K4Ge9 and alloys with similar nominal compositions yield only solid products with a ratio K:Ge ) 3:9, when “extracted” with [2.2.2]cryptand.6 It is also important to note that nido-Ge94- clusters prepared via the solid state as “neat” binary salts exhibit essentially C4V geometry.16 To date, the only evidence of an isolated Ge94- cluster drawn from solution was observed in Rb4Ge9‚en.17 However, the crystallographic studies were not (9) Xu, L.; Sevov, S. C. J. Am. Chem. Soc. 1999, 121, 9245. (10) Belin, C. H. E.; Corbett, J. D.; Cisar, A. J. Am. Chem. Soc. 1977, 99, 7163. (11) (a) Belin, C.; Mercier, H.; Angilella, V. New J. Chem. 1991, 15, 931. (b) Fa¨ssler, T. F.; Hunziker, M. Inorg. Chem. 1994, 33, 5380. (12) Fa¨ssler, T. F.; Schutz, U. Inorg. Chem. 1999, 38, 1866. (13) Campbell, J.; Schro¨bilgen, G. J. Inorg. Chem. 1997, 36, 4078. (14) Kirchner, P.; Huttner, G.; Heinze, K.; Renner, G. Angew. Chem., Int. Ed. 1998, 37, 1664. (15) Fa¨ssler, T. F.; Hunziker, M.; Spahr, M. E.; Lueken, H.; Schilder, H. Z. Anorg. Allg. Chem. 2000, 626, 692. (16) (a) Queneau, V.; Sevov, S. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 1754. (b) von Schnering, H. G.; Baitinger, M.; Bolle, U.; CarrilloCabrera, W.; Curda, J.; Grin, Y.; Heinemann, F.; Llanos, J.; Peters, K.; Schmeding, A.; Somer, M. Z. Anorg. Allg. Chem. 1997, 623, 1037. (17) Somer, M.; Carillo-Cabrera, W.; Peters, E. M.; Peters, K.; von Schnering, H. G. Z. Anorg. Allg. Chem. 1998, 624, 1915.
10.1021/ic001107q CCC: $20.00 © 2001 American Chemical Society Published on Web 07/26/2001
4722 Inorganic Chemistry, Vol. 40, No. 18, 2001 conclusive with several germanium atoms exhibiting “nonpositive definite” thermal parameters.17 Thus, the ability to isolate the Ge94- anion from ethylenediamine-alloy solutions without the use of sequestering agents was deemed promising. Considering the above observations, our attempts to isolate higher charged Ge94- anion from K/Ge alloy-en solutions exploited trends in stability constants of the alkali metal complexes. In this work, it was presumed that the use of sequestering agents that result in alkali metal complexes with lower stability constants in en would allow the isolation of higher charged “notso-naked” cluster anions with direct cation-anion interactions. These interactions would lower the net charge of the cationanion “complex”, and presumably promote the stabilization of anions with high formal charges per atom, as in Ge94- ) Ge(-4/9). The necessary prerequisites were fulfilled by the use of diaza-crown ethers whose alkali metal complexes exhibit significantly weaker stability constants in en than cryptands.18 Herein we report the synthesis and structure of [K-(2,2)diaza[18]-crown-6]K3Ge9‚2en which features two-dimensional layers of [K3Ge91-] and a “not-so-naked” Ge94- anion with a C2V distorted polyhedra that lies intermediate between a D3h tricapped trigonal prism and a C4V monocapped square antiprism. Experimental Section Synthesis. The compound [K-(2,2)diaza-[18]-crown-6]K3Ge9‚2en (1) was obtained from the reaction of (2,2)diaza-[18]-crown-6 (1,4,10,13tetraoxa-7,16-diazacyclooctadecane) with potassium metal and a binary alloy with a nominal composition “K4Ge9” in “neat” en. All experimental handling and manipulations were carried out under an inert argon atmosphere within a glovebox with moisture levels < 0.1 ppm. All solvents were degassed, distilled, and dried accordingly. The (2,2)diaza-[18]crown-6 (1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) was purchased from Aldrich and further oven- and vacuum-dried before use. A binary alloy of composition K4Ge9 was prepared by hightemperature reactions of the pure elements within sealed Nb tubing. A 0.104 g sample of the precursor “K4Ge9” alloy was dissolved in “neat” ethylenediamine (3 mL) resulting in a brown-red solution. Subsequent addition and dissolution of 0.023 g (0.59 mmol) of potassium metal turned the opaque brown-red alloy-en solution to transparent and bright red. Identical results can be observed by the addition of a saturated (gold-colored) K-en solution to the brown-red alloy-en solution. A separate solution of 0.067 g (0.255 mmol) of (2,2)diaza-[18]crown-6 was prepared in 4 mL of “neat” toluene. Subsequently, aliquot portions (0.3 mL) of each K-alloy/en solution were slowly layered with (0.3 mL) portions of the diaza-crown/toluene solution. The formation of large transparent bright-red platelike crystals of 1 was observed within 24 h at room temperature. Consequently, compound 1 was obtained in high yields (>75%). Structure Determination. Single-crystal X-ray analyses were performed on high-quality crystals of compound 1 formed from slow diffusion reactions. The single crystals were extremely air, moisture, and heat sensitive and were hence preserved in a mother liquor. Single crystals of 1 were subsequently isolated under mineral oil and Ar and mounted on glass fibers protected under Ar gas. The mounted crystal was then quickly mounted on a goniometer under the diffractometer cold stream (liquid N2). Diffraction intensities were measured, at -50 °C, on a Siemens SMART platform diffractometer with a 1K CCD area detector using Mo KR radiation. A hemisphere of data was collected using a narrowframe method with scan widths of 0.30° in ω and an exposure time of 30 s/frame. The first 50 frames were remeasured at the end of data collection to monitor instrument and crystal stability, and the maximum correction on I was 2σ(I)] R indices (all data) largest diff. peak and hole
K4Ge9C16H22N6O4 1192.27 223(2) P21/m 10.7398(9) 15.8121(13) 12.3259(10) 114.7380(10) 1901.1(3) 2 0.710 73 2.083 mg/m3 7.478 1.024 R1 ) 0.0318; wR2 ) 0.0842 R1 ) 0.0410; wR2 ) 0.0898 0.880 and -0.648 e Å3
GOF ) [∑(w(Fo2 - Fc2)2)/(n - p)]1/2. R1 ) (∑||Fo| - |Fc||)/ (∑|Fo|). wR2 ) [∑w(|Fo| - |Fc|)2/∑w|Fo|2]1/2. a
polarization, air absorption and absorption due to variation in the path length through the detector faceplate. Empirical absorption correction was applied on the hemisphere of data and redundant reflections averaged. The space group was shown to be P21/m with the following final cell constants a ) 10.7398(9) Å; b ) 15.812(1) Å; c ) 12.3259(10) Å, β ) 114.738(1)°; V ) 1901.1(3) Å3. Relevant data collection and structural refinement information are listed in Table 1. The structure was solved by direct methods and refined by full-matrix least-squares calculations and refined to final R indices of [I > 2σ(I)]: R1 ) 0.0318, wR2 ) 0.0842; [all data]: R1 ) 0.0410, wR2 ) 0.0898, GOF ) 1.024. Hydrogen atoms were located from a Fourier difference map and refined isotropically using a riding model (d ) 0.9 Å). The final Fourier difference map was essentially featureless. The refinement also indicated that one en molecule is disordered 50:50 over two slightly shifted orientations. The list of relevant bond distances and angles is given in Table 2.
Results and Discussion In our synthetic work, attempts to isolate Ge94- anion clusters with [2.2.2]cryptand from brown-red and bright-red alloy solutions yielded only crystalline salts containing Ge93- and [K4Ge18]2- clusters.19 Moreover, the yields of the cluster anions were observed to be lower for reactions involving the potassiumricher bright-red alloy solutions. Dissolution of “less-reduced” alloys with germanium-richer compositions (e.g., “KGe4”) in en resulted in dark green alloyen solutions. Isolation of Ge9 clusters from the green alloy solutions, using [2.2.2]cryptand, produced predominantly Ge92anions, as well as very low yields of isolated Ge93- and dimeric [K4Ge18]2- anions. The reaction of [18]crown-6 with the dark green alloy-en solutions affords the [Ge9]2- polymer anions.8 Consequently, the seemingly reversible color changes (greento-brown-to-red) of the alloy en solutions can be induced by the addition of appropriate amounts of potassium metal. These observations strongly imply that the color of alloy-en solutions and the seemingly reversible changes they exhibit are useful indicators of the degree of reduction (bright red) or oxidation (forest green) of the germanium anion clusters in en. Further electrochemical and spectroscopic studies are currently in progress to elucidate the solution chemistry of the Ge9 anions. The title compound [K-(2,2)diaza-[18]-crown-6]K3Ge9‚2en (1) crystallizes in the monoclinic space group P21/m. X-ray (19) Downie, C.; Guloy, A. M. Unpublished work.
Synthesis of [K+-(2,2)diaza-[18]-crown-6][K3Ge9]‚2en
Inorganic Chemistry, Vol. 40, No. 18, 2001 4723
Table 2. Selected Bond Lengths [Å] and Angles [deg] for [K(2,2) diaza-[18]-crown-6}K3Ge9‚2en Ge(1)-Ge(3) (2×) Ge(2)-Ge(4) (2×) Ge(2)-Ge(3) Ge(3)-Ge(6) (2×) Ge(4)-Ge(5) (2×) K(1)-Ge(6) K(2)-Ge(6) K(2)-Ge(3) K(2)-Ge(5) K(2)-Ge(3) K(1)-K(2) K(3)-K(2) K(2)-K(3) K(1)-N(1) K(1)-O(2) K(2)-N(3) K(3)-N(4′) K(3)-N(3)
2.5720(6) 2.6046(7) 2.7197(6) 2.5740(6) 2.5731(6) 3.434(1) 3.3998(8) 3.5977(9) 3.5224(9) 3.6117(9) 4.422(1) 4.2808(13) 4.2808(13) 2.882(4) 2.938(3) 3.032(5) 2.699(11) 3.411(6)
Ge(1)-Ge(2) (2×) Ge(2)-Ge(5) Ge(2)-Ge(2) Ge(3)-Ge(5) Ge(5)-Ge(6) (2×) K(1)-Ge(5) K(2)-Ge(2) K(2)-Ge(5) K(3)-Ge(4) K(3)-Ge(6) K(1)-K(2) K(2)-K(2) K(1)-N(2) K(1)-O(1) K(2)-O(1) K(2)-C(2) K(3)-N(4) K(3)-C(8′)
2.5964(7) 2.6267(6) 2.9087(9) 2.6136(6) 2.5858(6) 3.959(1) 3.6941(9) 3.5019(9) 3.568(2) 3.492(2) 4.422(1) 4.108(2) 2.793(5) 2.923(3) 2.912(3) 3.535(5) 2.888(9) 3.482(14)
Ge(6)-K(1)-Ge(5) Ge(6)-K(2)-Ge(5) Ge(5)-K(2)-Ge(5) Ge(5)-K(2)-Ge(3) Ge(6)-K(2)-Ge(3) Ge(5)-K(2)-Ge(3) Ge(6)-K(2)-Ge(2) Ge(5)-K(2)-Ge(2) Ge(3)-K(2)-Ge(2) Ge(3)-Ge(1)-Ge(3) Ge(3)-Ge(1)-Ge(2) Ge(3)-Ge(1)-Ge(2) Ge(1)-Ge(2)-Ge(4) Ge(4)-Ge(2)-Ge(5) Ge(4)-Ge(2)-Ge(3) Ge(1)-Ge(2)-Ge(2) Ge(5)-Ge(2)-Ge(2) Ge(1)-Ge(3)-Ge(6) Ge(6)-Ge(3)-Ge(5) Ge(6)-Ge(3)-Ge(2) Ge(5)-Ge(4)-Ge(5) Ge(5)-Ge(4)-Ge(2) Ge(5)-Ge(4)-Ge(2) Ge(4)-Ge(5)-Ge(6) Ge(6)-Ge(5)-Ge(3) Ge(6)-Ge(5)-Ge(2) Ge(3)-Ge(6)-Ge(3) Ge(3)-Ge(6)-Ge(5) Ge(3)-Ge(6)-Ge(5)
40.16(2) 43.97(1) 108.41(2) 93.96(2) 42.94(1) 93.37(2) 177.38(3) 42.61(1) 135.88(3) 71.31(2) 101.93(2) 63.50(2) 107.89(2) 58.93(2) 103.19(2) 55.93(1) 95.04(1) 105.76(2) 59.79(2) 102.87(2) 81.81(2) 104.24(2) 60.96(2) 97.84(2) 59.34(2) 105.17(2) 71.25(2) 60.87(2) 105.79(2)
Ge(5)-K(1)-Ge(5) Ge(6)-K(2)-Ge(5) Ge(6)-K(2)-Ge(3) Ge(5)-K(2)-Ge(3) Ge(5)-K(2)-Ge(3) Ge(3)-K(2)-Ge(3) Ge(5)-K(2)-Ge(2) Ge(3)-K(2)-Ge(2) Ge(6)-K(3)-Ge(4) Ge(3)-Ge(1)-Ge(2) Ge(3)-Ge(1)-Ge(2) Ge(2)-Ge(1)-Ge(2) Ge(1)-Ge(2)-Ge(5) Ge(1)-Ge(2)-Ge(3) Ge(5)-Ge(2)-Ge(3) Ge(4)-Ge(2)-Ge(2) Ge(3)-Ge(2)-Ge(2) Ge(1)-Ge(3)-Ge(5) Ge(1)-Ge(3)-Ge(2) Ge(5)-Ge(3)-Ge(2) Ge(5)-Ge(4)-Ge(2) Ge(5)-Ge(4)-Ge(2) Ge(2)-Ge(4)-Ge(2) Ge(4)-Ge(5)-Ge(3) Ge(4)-Ge(5)-Ge(2) Ge(3)-Ge(5)-Ge(2) Ge(3)-Ge(6)-Ge(5) Ge(3)-Ge(6)-Ge(5) Ge(5)-Ge(6)-Ge(5)
50.37(2) 136.31(3) 137.94(3) 43.06(1) 43.08(1) 110.52(2) 137.64(3) 43.77(1) 159.34(5) 63.50(2) 101.93(2) 68.13(3) 107.37(2) 57.81(2) 58.50(1) 56.06(1) 90.94(1) 108.51(2) 58.69(2) 58.97(2) 60.96(2) 104.24(2) 67.89(3) 107.13(2) 60.11(2) 62.53(2) 105.79(2) 60.87(2) 81.32(2)
single-crystal structure determination confirms that 1 contains four potassium ions for one unique Ge9 cluster and agrees with a negative charge of 4 for the polyatomic anion. The crystal structure, as shown in Figure 1, features a layered structure consisting of salt-like intermetallic slabs of [K3Ge91-] alternately stacked with and isolated by “organic” layers of “clustercapping” K+-(2,2)diaza-[18]-crown-6 complex cations (K1) and en molecules. The layered [K3Ge91-] structure can be described in terms of nominal “ion-pair” [K2Ge9] chains, running along b, as building units. The “ion-pair” [K2Ge9] chains are illustrated in Figure 2. The “not-so-naked” Ge94- clusters in the chain are linked through a pair of potassium ions (K2) that cap triangular faces of neighboring clusters. The bridging role of the K2 atoms in the “ion-pair” [K2Ge9]2- chain is also observed for Cs atoms that “link” [Cs4Ge18]2- clusters in [K-[2.2.2]crypt]2[Cs4Ge18]‚ 6en.9 Connecting parallel neighboring “ion-pair” [K2Ge9]2- chains along the a axis results in the layered structure of 1, as shown in Figure 3. The “ion-pair” chains are connected in two ways: (a) through potassium atoms (K3) that join adjacent Ge9 clusters of neighboring chains and (b) through en solvent molecules that coordinate adjacent K2 ions of neighboring [K2Ge9]2- chains. A second en molecule, disordered over two slightly shifted (“rocking”) orientations, coordinates every chain-linking K3
Figure 1. A [100] view of the unit cell of 1. Ge and K atoms are represented as crosshatched and open ellipsoids, respectively. Lighter atoms (C, N, O) are represented as small circles. Ellipsoids are drawn at 60% probability. For clarity, only one orientation of the disordered en solvent molecule is drawn.
cation in a bidentate manner. The coordination of K3 by en effectively fills-in the gaps between linked “ion-paired” chains. Each complex cation [K+-(2,2)diaza-[18]-crown-6] caps the “exposed” open base of the Ge94- clusters. Thus, the bidentate en molecules and the complex cation practically sheath and isolate the [K3Ge91-] slabs. Additional stabilization of the layered anion formation comes from the interaction between the oxygen atoms of the sequestering agent and face-bridging potassium (K2) manifested by the short O-K distances. The important feature of 1 is the Ge94- anion, shown in Figure 4. The excellent structural refinement of 1, with low standard deviations, allows for a definitive geometric description of the Ge94- anion. The crystal structure reveals that the Ge94- anion exhibits Cs symmetry. Inspection of the bond distances, dihedral angles, and diagonal distances leads to the conclusion that the 22-electron cluster is approximately C2V and close to D3h, contrary to Wade’s rules.20 When compared with known nidoGe9 clusters, the anion in 1 is significantly distorted from C4V.8-10,16,17 A list of known distances and Ge9 cluster dimensions is given in Table 4 for comparison. The calculated height (h)/edge (e) ratio of 1.17, prism heights (2.909(1) Å, 2.998(1) Å, and 3.370(1) Å), and dihedral angles for the nominal tricapped trigonal prism (11°, 23°, and 26°) illustrate the significant deviations of the Ge94- anion in 1 from C4V symmetry. Furthermore, the diagonal distances in the nominal “square base” for a monocapped square antiprism in 1 are significantly unsymmetrical (d1/d2 ratio ) 1.16). Thus, the geometry of the Ge94- cluster compares well with that of known Ge93- anions, found in Ge cluster compounds with crystallographically unique cluster units, which are essentially closer to being tricapped trigonal prism (D3h).10-12 The possibility of having a Ge93- anion is discounted by the presence of one unique cluster and confirmed by magnetic susceptibility measurements which indicate that compound 1 is diamagnetic. The observed deviations from ideal C4V values may be manifestations of the layered nature of the crystal structure and (20) Wade, K. AdV. Inorg. Chem. Radiochem. 1976, 18, 1.
4724 Inorganic Chemistry, Vol. 40, No. 18, 2001
Downie et al.
Figure 2. ORTEP representation of the “ion-paired” [K2Ge9]2- chain in 1. Ge and K2 atoms are represented as crosshatched and open ellipsoids, respectively. Distances between K2 and Ge that range from 3.400(1) to 3.694(1) Å are drawn for clarity. Ellipsoids are drawn at 90% probability. Table 3. Relevant Geometrical Parameters in Some Isolated Ge9nClusters and Related Nine-Atom Clusters prism heights cluster 4-
Ge9 Ge94Ge94Ge94Ge94Ge94Ge92Ge93Ge93Bi95+ Bi95+ Sn94Sn94Pb94Si94a
Figure 3. ORTEP representation of the [K3Ge9]1- slab in 1. Ge and K atoms are represented as crosshatched and open ellipsoids, respectively. Lighter atoms (C and N) are represented as small circles. Distances between Ge and K atoms are drawn as thin solid lines. Coordination of K2 by en is drawn as dashed lines for clarity. Ellipsoids are drawn at 60% probability.
Figure 4. ORTEP representation of the unique Ge94- cluster in 1. Ellipsoids are drawn at 50% probability. Bond lengths [Å] within the cluster: Ge1-Ge2 2.5964(7), Ge1-Ge3 2.5720(6), Ge2-Ge2 2.9087(9), Ge2-Ge3 2.7197(6), Ge2-Ge4 2.6046(7), Ge2-Ge5 2.6267(6), Ge3-Ge3 2.9980(7), Ge3-Ge5 2.6136(6), Ge3-Ge6 2.5740(6), Ge4Ge5 2.5731(6), Ge5-Ge6 2.5858(6). The distance Ge5-Ge5, represented by a dashed line, is 3.3702(6) Å.
can be attributed to the direct cation-anion interactions of the “not-so-naked” cluster compound and to the “softness” of the Ge94- anion. Moreover, the crystal packing effects of nonspherical complex cations and co-crystallized solvent molecules may play significant roles in the local symmetry of the anionic Ge9 clusters in the solid state. However, a nido-E94- that exhibits nearly D3h geometry can also be rationalized in terms of electronic effects analogous with those observed for the Bi95+ cation and Sn94- anions in [K4Sn9([18]-crown-6)3] and [K4Sn9([18]-crown-6)4].6a,21-22 Furthermore, inspection of cluster geometry parameters in the solid-state compounds K4Pb9, Cs4-
symmetry
h1
h2
h3
h/e
R1a
ref
Cs (∼D3h) ∼C4V ∼C4V ∼C4V ∼C2V ∼C4V C2V (∼D3h) Cs (∼D3h) Cs (∼D3h) D3h ∼D3h ∼C2V (∼D3h) ∼C2V (∼D3h) C2V (∼D3h) ∼D3h
2.90 2.83 2.80 2.81 2.87 2.78 2.81 2.81 2.87 3.74 3.70 3.32 3.35 3.45 2.67
3.00 2.88 2.99 3.00 3.01 2.87 2.84 3.21 3.15 3.74 3.70 3.45 3.38 3.90 2.94
3.37 3.62 3.54 3.58 3.41 3.47 3.21 3.27 3.33 3.74 3.98 3.54 3.74 3.90 3.08
1.17
11 4 6 7 11 5 8 14 16 22 16 15 13 17 19
this work 24b 24b 24b 24b 10 10 11a 11b 21a 21b 22 22 23 24a
1.17 1.11 1.17 1.17 1.15 1.19 1.14 1.15 1.16 1.16
Angle R1 is the smallest “cap-to-cap” dihedral (waist) angle.
Ge9, and Rb12Si17 indicate that at least one type of Pb94-, Ge94-, or Si94- cluster anion in each compound, respectively, exhibits significant distortion toward D3h symmetry.6a,23-24 These ions form a separate group of nine-atom 22-electron clusters that mainly exhibit elongated trigonal prisms and larger height-toedge (h/e) ratios that correlate with the stabilization of the a′′ orbital (HOMO for 22-electrons) of a D3h (E9)-cluster.6a,22-24 The relevant molecular orbital is found to be antibonding with respect to the trigonal prism lengths and bonding with respect to the triangular faces of the trigonal prism. Comparisons of geometric ratios indicate close similarities between the unique anion in 1 with non-Wade’s rule clusters Bi95+, Sn94-, Ge94(1 of 4 clusters in Cs4Ge9), and Pb94- (1 of 2 clusters in K4Pb9).21-23 Hence, the Ge94- anion in 1 may be considered a representative of this group of non-Wade’s rule clusters. Thus, the findings of this work prove that assigning charge allocation to Ge9n- anions solely based on geometric parameters is insufficient. Our findings support earlier proposals that the first steps of “alloy” solvation in en does not involve the immediate breaking of the intimate close contact between cations and the distinct anionic clusters.17 Our results suggest that the solvation process is an “exfoliation” of the alloy structure into lower-dimensional derivative “ion-pair” structures of “not-so-naked” anions with lower net negative charges. Of course, the isolation and crystallization of large unligated anion clusters depend on several rather complex and subtle factors (e.g., ionic sizes, cation-cation and anion-anion (21) (a) Friedman, R. M.; Corbett, J. D. Inorg. Chem. 1973, 12, 1134. (b) Hershaft, A.; Corbett, J. D. Inorg. Chem. 1963, 2, 979. (22) Fa¨ssler, T. F.; Hoffmann, R. Angew. Chem., Int. Ed. 1999, 38, 543. (23) Queneau, V.; Sevov, S. Inorg. Chem. 1998, 7, 358. (24) (a) Queneau, V.; Todorov, E.; Sevov, S. C. J. Am. Chem. Soc. 1998, 120, 3263-3264. (b) Queneau, V.; Sevov, S. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 1754-1756.
Synthesis of [K+-(2,2)diaza-[18]-crown-6][K3Ge9]‚2en interactions, anion-cation interactions, nature of the solvent, solute-solvent effects). However, the successful use of noncryptands (e.g., [18]-crown-6 and (2,2)diaza-[18]-crown-6) with weaker sequestering abilities, in addition to controlled concentration, significantly add new synthetic handles for investigating the important chemistry of polyanionic clusters in ethylenediamine and in similar basic solvents. Acknowledgment. Financial support from the Petroleum Research Fund, the Welch Foundation, and the National Science
Inorganic Chemistry, Vol. 40, No. 18, 2001 4725 Foundation (CAREER Award, DMR-9733587) is gratefully acknowledged. The work made use of MRSEC/TCSUH facilities at the University of Houston supported by the NSF (DMR9632667). Supporting Information Available: X-ray crystallographic file in CIF format for the structure determination of [K-(2,2)diaza-[18]-crown6]K3Ge9‚2en. This material is available free of charge via the Internet at http://pubs.acs.org. IC001107Q
